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Evolution
Like all living things, animals undergo changes as each new generation succeeds the one before. These changes are usually so slight that they are very difficult to see, but over thousands or millions of years, they can completely alter the way animals look and also the way they behave. This process of change is called evolution. It allows animals to exploit new opportunities and to adapt to changes that take place in the world around them. Evolution works by modifying existing characteristics, usually through a series of extremely small mutations. The result of this is that every animal is a living store of evolutionary history - one that helps to show how different species are related. Animal Adaptation Evolution is made possible by the variations that exist within animals, and it occurs mainly because animals compete with each other for limited resources, such as space and food. In this competition, some characteristics prove to be more useful than others, which means that their owners are more likely to thrive and to produce the most young. Animals with less useful features face more difficulties and find it harder to breed. The least successful animals are therefore gradually weeded out, while those with "winning" characteristics become widespread. This weeding-out process is called natural selection. Species and Speciation Extinction Convergence Biogeography Periods Geologists divide earth's history into periods characterized by major physical changes, such as bursts of volcanic activity, collisions between continents, or alterations in climate. Many of these periods have ended in worldwide mass extinctions, both on land and in the sea. For each period described below, the date shown indicates the period's end. *''Precambrian (542 million years ago):'' This vast expanse of geological time stretches from when the continents first formed to when animals with hard body parts first appeared in the fossil record. Life itself emerged near the beginning of Precambrian time, about 3.8 billion years ago. The date for the first animals is less certain: being soft-bodied, they left few traces, although their existence is evident from fossils of burrows and tracks (about one billion years old). *''Cambrian (488 million years ago):'' The start of the Cambrian period was marked by an extraordinary explosion of animal life in the seas. Cambrian animals included mollusks, echinoderms, and arthropods. These were among the first creatures to have hard, easily fossilized body parts. By the end of this period, all of the major divisions of animal life, or phyla, that exist today had been established. *''Ordovician (444 million years ago):'' *''Silurian (416 million years ago):'' Silurian times saw the evolution of the first fish with jaws and of giant sea scorpions - relatives of today's arachnids. The first land plants appeared. *''Devonian (359 million years ago):'' *''Carboniferous (299 million years ago):'' *''Permian (251 million years ago):'' *''Triassic (200 million years ago):'' *''Jurassic (145 million years ago):'' *''Cretaceous (65 million years ago):'' *''Tertiary (1.8 million years ago):'' *''Quaternary (present):'' This period has seen abrupt changes in climate. Mammals have become supreme, and humans have become the dominant form of life. Category:Unfinished pages